1. Field of the Invention
The present invention relates to a thermosensitive recording material utilizing a color forming reaction of an electron donating coloring agent and an electron accepting color developer and an image forming method therefor, and more particularly to a thermosensitive recording material for thermal recording using a laser which is useful as block copy films for flexography, gravure printing, offset printing and screen printing.
2. Discussion of the Related Art
Thermal recording is well known in which color images can be formed by imagewise heating with a thermal printhead a thermosensitive recording layer, or a protective layer formed on the recording layer, of a thermosensitive recording material. Such thermal recording is widely utilized for image forming apparatus such as facsimiles, printers for measuring instruments and the like. The thermal recording using a thermal printhead has the following drawbacks:
(1) undesired images such as white streaks and omissions of images tend to be formed on recording materials because dust easily adheres to thermal printheads or thermal printheads easily stick to the recording materials; and PA1 (2) thermal printheads are easily worn or damaged, resulting in deterioration of image qualities. PA1 providing the thermosensitive recording material mentioned above; and PA1 irradiating the recording material with a laser having a wavelength of from about 500 to about 550 nm or from about 780 to about 850 nm, or with a Nd:YAG laser, to form images on the recording material.
In particular, when a long image is recorded or many images are continuously recorded with a thermal plotter using a thermal printhead, it is almost impossible to obtain a good image or images without undesired images such as white streaks and omissions of images. In addition, thermal recording using a thermal printhead has drawbacks in that images having high resolution cannot be obtained and high speed recording is impossible because there are structural or physical limits of dot density and heating/cooling cycle times to heating elements of thermal printheads.
In addition, when thermosensitive recording materials in which images are formed are used as block copies for printing such as screen printing, it is especially desired to prevent the undesired images mentioned above from occurring and to improve image qualities such as resolution.
In attempting to solve these drawbacks of thermal recording using thermal printheads, Japanese Laid-Open Patent Publications Nos. 5-301447 and 9-20021 have disclosed thermal recording methods using a laser as a writing medium, which can produce images having high resolution at a high recording speed without touching the surface of a thermosensitive recording material.
However, the thermal recording method using a laser has a drawback in that good images cannot be obtained unless the output power of the laser is relatively large, because thermosensitive recording materials tend not to absorb near infrared light or infrared light of a laser. Therefore, it is difficult to provide a compact and low-cost thermal recording apparatus which uses a laser as a writing medium.
In attempting to solve the drawback of the thermal recording using a laser, thermosensitive recording materials are proposed which include a substance, which can effectively absorb the laser light, in a recording layer or a layer near the recording layer.
For example, Japanese Laid-Open Patent Publication No. 2-209290 discloses a thermosensitive recording material in which a light absorbing layer is formed on a recording layer; Japanese Laid-Open Patent Publications Nos. 3-86581 and 4-141485 disclose recording methods in which a laser irradiates a sheet which includes a light absorbing agent such as carbon black and which is overlaid on a thermosensitive recording material, to form images on the recording material; Japanese Laid-Open Patent Publications Nos. 4-357080 and 4-357082 disclose thermosensitive recording materials including a light absorbing agent which is microencapsulated; Japanese Laid-Open Patent Publication No. 2-120082 discloses a thermosensitive recording material including an infrared absorbing layer which is formed under a recording layer; and Japanese Laid-Open Patent Publications Nos. 5-278329, 6-72028, 7-186546, 8-187945, 8-90919, 8-127180, 8-187947, 8-258420 and 8-267920 disclose thermosensitive recording materials including a laser light converting material which can absorb laser light to convert the light energy to heat. However, images having good image qualities cannot be obtained by these recording materials or methods.
Laser light having a near infrared or infrared wavelength is typically used for thermal recording because recording materials color if a light absorbing substance absorbing visual light, i.e., a colored light absorbing substance, is used in the recording material. Therefore light absorbing agents used for recording materials for laser recording are limited and generally have high costs, resulting in increase of manufacturing costs of the recording materials. In addition, a problem tends to occur in that human eyes are damaged by infrared laser light because the laser light cannot be seen.
Because of these reasons, a need exists for a thermosensitive recording material which can produce images having good image qualities such as high resolution using a laser as a writing medium and which can preferably be used as block copies for printing.